The U.S. Pat. No. 7,775,101B2 discloses an apparatus for measuring a manufacturing deviation in an external helical gear, in which a gear to be checked is arranged onto a stationary central pin, the external diameter of which is smaller than the internal diameter of the gear to be checked so that a relative movement between the gear to be checked and the pin is allowed. Moreover, the pin has a reference ball, which partially protrudes from a lateral surface of the pin and leans, in use, against an internal lateral surface of the gear to be checked. Under the stationary central pin, a lower face of the gear to be checked leans initially on a front rest ball, which partially protrudes from a support plane and is placed next to the reference ball, and on two back pads, the height of which above the support plane is lower than the one of the rest ball.
In use, a master gear (a master, that is a calibrated gear which is manufactured in an extremely accurate way and made of a very hard material) with external teeth analogous to the external teeth of the gear to be checked is laterally pushed against the gear to be checked with a predetermined strength (provided by an elastic member) to mesh with the gear to be checked.
When the two gears mesh, the gear to be checked is slightly lifted and its lower face continues to lean only on the front rest ball (that is, it doesn't contact the two back pads anymore).
Once the master gear and the gear to be checked are meshed as hereinbefore described, an electric motor mechanically connected to the master gear puts into (slow) rotation the master gear in order to put in rotation the gear to be checked, too, owing to the meshing between the two teeth. According to the check, the gear to be checked has to perform at least one complete rotation so that all the teeth of the gear to be checked mesh with the teeth of the master gear at a given instant of time of the check (obviously, it is possible that the gear to be checked performs more complete rotations in order to get a redundancy of data to average and limit the incidence of accidental errors).
The pin carrying the gear to be checked includes two groups of position sensors (typically two sets of three position sensors arranged symmetrically about the central axis), which are arranged along the pin at two different levels (that is, the two groups of sensor devices are staggered along the lateral surface of the pin). In each group of position sensors, the position sensors are all placed at the same level (that is, the position sensors are coplanar) and lie in a measuring plane perpendicular to the external lateral surface of the pin and to the internal lateral surface of the gear to be checked. Each position sensor is adapted to measure a distance lying between the external lateral surface of the pin and the internal lateral surface of the gear to be checked. Knowing the distance lying between the external lateral surface of the pin and the internal lateral surface of the gear to be checked at three different points of a same measuring plane, it is possible to determine in the same measuring plane the position of the central axis of the gear to be checked. Knowing the position of the central axis of the gear to be checked in two different measuring planes, it is possible to determine the position of the central axis of the gear to be checked. During a complete rotation of the gear to be checked, the real position of the central axis of the gear to be checked is cyclically determined according to the above-described procedure, and it is possible to accurately determine, at each point of the gear to be checked, the taper error and the lead error as a function of the deviation between the real position and the ideal position of the central axis of the gear to be checked.
The above-described apparatus enables to determine the taper error and the lead error with good accuracy in a workshop environment. However, the accuracy standards required are higher and higher.
Moreover, the above-described apparatus is relatively cumbersome, expensive and not very adaptable due to the need to employ the master gear, which is put into rotation by a servomotor. It should be noted that the master gear is particularly difficult and expensive to produce because on the one hand it has to be very accurately manufactured and on the other hand it has to be made of very hard material. Moreover, as the master gear has to be conjugated with the gear to be checked, each type of gear to be checked requires a corresponding master gear.